Metabolism in a living body is an extremely efficient reaction with a high substrate specificity which proceeds in a comparatively mild environment (neutral at room temperature). Such metabolism includes respiration and photosynthesis that converts nutrients (such as oxygen, saccharides, fats, and proteins) into energy required for growth of microbes and cells.
Biocatalysts (or enzymes) composed of proteins get deeply involved in such reactions in a living body. The idea of utilizing the catalytic action of enzymes had been put to practice in the long human history. The application of enzymes covers various fields such as brewing industry, fermentation industry, textile industry, leather industry, food industry, and pharmaceutical industry. Enzymes are expected to find new uses in the field of electronics, such as biosensors, bioreactors, and bio-fuel cells, which have electrodes catalyzed by enzymes.
Unfortunately, enzymes have been used exclusively in an aqueous medium because they are proteins which are unstable to heat, strong acid and alkali, and organic solvent. In the past, the enzymatic reaction has been carried out by the batchwise process that causes enzymes dissolved in an aqueous medium to react on the substrate. The batchwise process is uneconomical because it is repeated after enzymes have been discarded. In fact, it is very difficult to recover enzymes intact (for reuse) from reaction solutions.
To address this problem, there have been proposed immobilized enzymes, which are insoluble in water. Immobilized enzymes (with high substrate specificity) can be used in the same way as solid catalysts for ordinary chemical reactions. Immobilization is a highly effective way of using enzymes.
The same is true for the application of enzymes to electrodes. Enzymes densely immobilized on the surface of an electrode produce enzymatic reactions near the electrode and such enzymatic reactions can be detected as electric signals. Incidentally, an electron mediator (or electron acceptor) is necessary between the enzyme (protein) and the electrode to promote electron transfer, and this electron mediator should also be immobilized preferably.
There are generally two methods for immobilizing enzymes on electrodes—the entrapping method and the bonding method. Research is progressing on how to immobilize enzymes on various electrode materials.
According to the related arts, electrode materials which have preferentially been used for high reaction efficiency are carbonaceous porous ones with a large surface area. (See, Japanese Patent Laid-open Publication (JP-A) No. 2000-133297, JP-A No. 2003-282124, JP-A No. 2004-71559 and JP-A No. 2005-13210.) Carbonaceous porous electrode materials, however, have a very small pore diameter and are limited in porosity (which affects strength). Consequently, they prevent a solution (containing enzymes or substrate for reactions) from infiltrating into them, resulting in uneven distribution of enzymes and substrate. That is, the advantage of their high surface area has not been fully utilized. This problem is more serious when a highly viscous solution is used or the enzymatic reactions involve a large pH change. In these cases, the solution does not infiltrate into the inside and the buffering function does not follow the abrupt pH change in the electrode, which would lead to enzyme deactivation.
Much has been studied about immobilization of enzymes on carbonaceous materials as well as metallic materials such as titanium, copper, aluminum, nickel, stainless steel, chromium, gold, and platinum. (See, Japanese Patent Laid-open Publication (JP-A) No. 2000-133297, JP-A No. 2003-282124 and JP-A No. 2004-71559.) However, metallic materials are poor in stability (or liable to corrosion and dissolution depending on solution pH and potential) and inferior in surface area to carbonaceous materials.
Although carbonaceous materials as well as, metallic materials have been used as raw materials for electrodes on which enzymes are to be immobilized, they have their merits and demerits, as mentioned above.
It is desirable to provide a porous electroconductive material and a process for production thereof; an electrode made of the electroconductive material and a process for production thereof; a highly efficient fuel call equipped with the electrodes on which enzymes are immobilized and a process for production thereof; and an electrode reaction-based apparatus equipped with the electrodes having immobilized enzymes thereon the porous electroconductive material is characterized by adequate pore diameters (large enough for a solution containing the substrate to easily pass through), high porosity, high conductivity, and large surface areas. Moreover, the porous electroconductive material enables efficient enzymatic metabolic reactions on electrodes and yields electrodes having immobilized enzymes thereon which remain stable in any working environment.
It is further desirable to provide an electronic instrument, a mobile machine, an electric power generating system, and a cogeneration system, which are equipped with the highly efficient fuel cell.